Electric device and process of making the same



J. c. RAH

June 1, 1937.

ELECTRIC DEVICE AND PROCESS OF MAKING THE SAME Fil ed Aug. so, 1935 Patented June 1, 1937 I UNETED STATES 2,.ii8h027 PATENT OFHCE ELECTRIC DEVICE AND PROCESS OF MAK- ING THE SAME corporation of Illinois Application August 30,

'7 Claims.

In the pending application of Elias S. Cornell, Charles A. Koerner, and Joseph A. StoOs, Serial No. 685,048, filed August 14, 1933, there is shown a transfer stud which is used on a circuit breaker in connection with other apparatus for effecting a selective connection between one pole of the breaker and either one of two sets of con tacts. The present invention is concerned with the construction and method of insulating the 10 transfer stud.

The transfer stud consists, in its essence, of a pair of spaced switch terminals joined by a conducting bar and enclosed, except for the contacting ends of the terminals, in a body of insulation which is molded around the same. The insulation preferably comprises a phenol condensation production known as bakelite.

In the design of a stud of this type, primary consideration must be given to two important problems arising from the nature and use of such a device. Firstly, the mechanical problems which arise in connection with the application of insulating materials to an electrical conductor in order to efiect a perfect and permanent adhesion therebetween and yet to provide sufficient elasticity throughout the life of the device to eliminate the destructive effect of the differences in coefficient of expansion of the various materials, must be overcome by proper selection, design 30 and application of the materials. Secondly, the

electrical problems which are involved in properly proportioning the various materials used in insulation, as such materials have different dielectric constants. In connection with this latter feature, for example, these materials must be so selected and proportioned that the entire assembly will withstand operating potentials running as high as 15,000 volts, and testing potentials running up to approximately 40,000 volts. Also, the design must be such that the maximum voltage gradient at any point in the insulating structure will be below the limit at which corona will form, since formation of corona discharges within the insulating structure will ultimately cause break down of the insulation. With these primary consideraticns in mind, thepresent invention is directed particularly to the provision of a terminal stud in which these problems have been adequately solved.

When plastic material, such as bakelite or the like, is molded around a solid conductor, difiiculties are encountered due to the unequal expansion and contraction of the molded material and the conductor under thermal changes. This 55 causes cracking of the molded material. It is 1933, Serial No. 687,457

an object of the present invention to overcome this difficulty. I propose to accomplish this result by coating the conductor with an elastic material, such as gum rubber, and molding the bakelite insulation around the coated conductor. By this arrangement the rubber constitutes a floating support for the conductor within its molded insulation and allows for relative expansion and contraction between the conductor and the molded insulation under thermal changes.

The gum rubber is far inferior to the molded material as an electric insulator, and it is not relied upon for that purpose. The molded insulation is made of a thickness necessary to supply the requisite insulation. When plastic bakelite is formed around rubber there is a tendency for, the rubber to penetrate into the bakelite under the heat and pressure prevailing during the molding process. The rubber tends to form needle-like projections extending into the body of the bakelite. Since the insulating properties of the rubber of each needlelike projection are far inferior to the insulating properties of the bakelite which it replaces, the net result of the formation of these projections is a reduction in the insulation afforded by the molded bakelite. This reduction is pro portionate to the extent of penetration or length of the longest projection, since the insulation of the entire piece is no greater than the insulation at the weakest part thereof. It is one of the objects of the present invention to prevent the penetration of the rubber into the bakelite during the molding process.

I have found that the outer surface or skin of a body of rubber may be made tougher than the main body of rubber. This tough skin tends to resist the tendency of the main body of rubber to flow into the plastic material under the pressure and temperature conditions prevailing during molding. However this skin is so thin that, while it resists the tendency to penetrate, it can not prevent it. To overcome this difiiculty, I propose to build up the rubber coating on the conductor in the form of a large number of very thin layers, each layer of which is treated to form a tough skin thereon before the next thin layer of rubber is applied. By this method I build up the rubber to the requisite thickness and of qualities such that it does not penetrate the molded material during the molding stage. However, it is to be noted that the rubber is not hardened or vulcanized by this process, as the use of a Vulcanized rubber is undesirable, since it has a pronounced tendency to disintegrate due til ' by the socket terminal.

- of rubber I6, in a manner to to the effect of corona discharges, and the uniformity of its dielectric characteristic is thus disturbed. On the other hand, the use of a softer, unvulcanized rubber as employed in the present invention eliminates, to a great extent, any initial or subsequent formation of air pockets or voids, thus reducing the possibility of corona and its resulting destructive effects.

Another object of the present invention is to prevent, as far as possible, any incipient corona, since if corona discharges are once allowed to be set up, their subsequent effect is cumulative, resulting in untimely breaking down of the insulation.

The attainment of the above and further objects of the present invention will be apparent from the following specification taken in conjunction with the accompanying drawing forming a part thereof.

Figure 1 is a view, in partial section, showing my improved transfer stud as part of an adapter which is mounted on the head of a circuit breaker; and

Figure 2 is a top view, in reduced scale, of the transfer stud.

Reference may now be had more particularly to Figure 1. For the purpose of illustration, I have shown at l a portion of the head of an oil circuit breaker. The circuit breaker includes a socket terminal 2 for establishing electrical connections to one side of the breaker, and a terminal stud 3 for establishing connections to the other side of the breaker. The socket terminal 2 is of a construction such as shown in the pending application of Alfred Alsaker and Fred H. Cole, Serial No. 681,158, filed July 19, 1933.

Briefly, the socket terminal comprises a large number of contact wires 4, swaged or otherwise suitably secured to a plug 5 that is threaded over a terminal stud 6 comprising one terminal of the circuit breaker. Coiled springs I, in the form of rings, embrace the contact wire assembly and urge the wires inwardly into firm engagement with a contact plug that is receivable A plurality of internal rings 3 limit the extent of inward collapse of the contact wires when the socket does not contain a plug. A sleeve 9, of suitable insulation, surrounds the contact socket for preventing accidental engagement therewith.

It is to be noted that the terminal stud 3 is appreciably below the terminal stud 6. When the adapter, to be presently described, is mounted in place it brings the overall height of the corresponding terminal of the circuit breaker to that of the socket terminal 2.

The adapter comprises a transfer stud I0 and a transfer plug. The transfer stud includes a copper bar I2, one end of which is secured to the stud 3. The other end of the copper bar carries a stud I3 that threads into a plug I5. The stud 3, which threads through the copper bar I2, also threads into the base of a copper plug I4. The copper bar I2 is covered with a layer be more fully set forth as the description proceeds, and is enclosed in a body of insulation I1 which is molded around it and may comprise any suitable insulation, such as the material known as bakelite. The body of insulation includes cylindrical pockets I8 and I9 formed integrally with the body of insulation IT. The ends of the copper bar I2 extend into the cylindrical pockets and divide each of these pockets into upper and lower cylindrical portions. The transfer stud includes a supporting leg comprising an insulating rod 20 and an insulating tubing 2I, and a mounting base 22. The rod 20 is secured in place by the stud I3, the lower end of which threads into the rod. The mounting base is secured to the rod 20' by means of a bolt 23 that threads into the rod 20 and holds the base thereto. The mounting base 22 is adapted to be bolted or otherwise secured to the head of the circuit breaker I.

The base of each of the contact plugs I4 and I5 is internally threaded for receiving the studs 3 and I3 respectively, as previously set forth. The stud 3 establishes the electric circuit between the circuit breaker and the copper bar I2, and also establishes a circuit to the plug I4. The stud I3 extends the electric circuit between the bar I2 and the plug I5. Tubes of insulation 24 and 25 are internally threaded at their lower ends and thread over the external thread on the bases of the plugs I4 and I5 respectively. These tubes are spaced from the plugs by an amount sufiicient to permit the entry of a socket terminal of a transfer plug.

The transfer plug is indicated at 30. It com prises a unitary body of insulation 3I including I a bridge member 32 having a cap 33 formed at one end thereof and a socket receiving cylindrical formation 34 at the opposite end. A pair of oppositely facing sockets 35 and 36, of a construction similar to the socket terminal 2, are secured to the portion 34 of the transfer plug. A tube of insulation 31 surrounds the socket 35, being fitted over a cylindrical portion of the main body of insulation comprising the transfer plug. A pair of spring clips H, transfer stud extend through a centrally located opening in the bridge portion 32 of the transfer plug and serve to snap the transfer plug into its fully closed position when it is brought approximately to its fully closed position.

The springs 404I are suitably secured to the opposite sides of an upwardly extending portion 42 of insulation formed integrally with the body of insulation H. In order to strengthen the body of insulation 42 the copper bar I2 is provided with a hole 43 immediately beneath the body of insulation 42. During the molding process the hole 43 is, of course, filled with insulation which constitutes an integral part of the insulation IT.

The insulation within this hole thus serves to take some of the stress imposed upon the main body of insulation I1 and prevents an overstressing of the comparatively thin insulation between the bar I2 and the outer surface of the body I1.

In the position shown in Figure 1 the transfer plug covers the plug I4, the socket 36 being in a position to engage the plug I5. This places the socket 35 at a given distance from the socket terminal 2. If desired, the transfer plug may be reversed so that the cap 33 covers the plug I5, and the socket 36 engages the plug I4. When this is done, the socket 35 is closer to the socket terminal 2 than when it is in the position indicated in Figure 1. In its two alternate positions, the socket 35 is adapted to engage different plugs for establishing different circuits. It is to be noted that in either position of the plug all of the live parts of the transfer stud are fully enclosed by adequate insulation.

An explanation will now be of forming the insulation around the copper bar I3. The copper bar is first freed of all sharp corners and projections, all of the corners being rounded, and is then thoroughly cleaned. The bar is then dipped into a vat containing pure carried by the i given of the mode ill) six'times until the desired thickness of rubber has been built up. I have found that a thickness of about a of an inch is sufficient. The last coat of rubber is baked for approximately two hours, after which it is gradually cooled to room temperature over a period of about twelve hours. This results in a thorough drying of the rubber. At the same time, since the rubber is not heated to its vulcanizing temperature, this rubber coating remains in a soft, plastic and unvulcanized state, whereby it is effective for accommodating the differences in expansion between the various materials, and at the same time eliminates, to a great extent, any formation of voids or air pockets, and thus reduces the possibility of incipient corona discharges. The bar, with its rubber coating, is then placed in a bakelite mold to form a bakelite enclosure for the bar. It is cured by baking for ten to fifteen minutes at 350 F. It is then aged outside of the mold by baking for approximately forty eight hours at 300 F. Thereafter it is coated with bakelite resin varnish, and the insulating rod 28 and the insulating tubing 2|, both also coated with the bakelite resin varnish, assembled in place. This assembly is then baked for four to six hours at 275 F. The length of this period of baking is determined by the time required to produce a high surface resistance, as determined by proper electrical tests.

The ageing outside of the mold for forty eight hours is for the purpose of causing the bakelite to shrink all that it will ever shrink. This assures that there will be no shrinkage of the bakelite during the life of the device. This overcomes the difficulty due to the tendency of bakelite to shrink as it ages, since such shrinkage may cause cracking of the bakelite, to the detriment of the device in service. By artificially ageing the bakelite I produce, in forty eight hours, all the shrinking that would normally take place during the entire life of the device. Those devices that crack during the ageing process can be rejected, thus assuring that the serviceable devices will remain serviceable during the life of the device.

When plastic bakelite is formed around a rubberized compound, there is a tendency for the rubber to penetrate into the bakelite in the form of needle-like projections extending into the body of the *bakelite. Since the rubber has poor insulating qualities, and is not relied upon for insulation, such penetration will reduce the insulating properties of the bakelite to a substantial amount and must, therefore, be prevented. By applying the rubber onto the copper bar in a number of very thin coats, the plasticizer forms a continuous surface around each layer and avoids the production of a spongy mass of rubber. The plasticizer, it appears, strengthens the outer coating of each layer and, by surface tension, serves to hold the rubber against penetrating into the bakelite under the action of the heat and pressure incident to the molding operation.

In compliance with the requirements of the patent statutes I have herein shown and described a preferred embodiment of my invention. It is, however, to be understood that the invention is not limited to the precise structure and method described, the same being merely illustrative of the principles of the invention.

What I consider new and desire to secure by Letters Patent is:'

l. The method of insulating an electrical conductor carrying substantial current and subjected to expansion due to heating by said current, which comprises superposing a plurality of enveloping layers of a solution of gum rubber and a plasticizer about said conductor, baking each of said layers at a temperature below the vulcanizing temperature of said rubber before application of the next succeeding layer, and enclosing said layers in a hard phenol condensation product casing by molding said casing thereabout under heat and pressure.

2. The method of enclosing an electrical conductor carrying substantial current and subjected to expansion due to heating by said current, within a hard molded insulating casing of high dielectric strength and of a different coeificient of expansion than that of said conductor, which comprises applying a plurality of successive layers of a solution consisting of gum rubber and a plasticizer about said conductor, drying each layer of said solution prior to application of the next succeeding layer at .a temperature below the hardening temperature'of said rubber, and molding a phenol condensation product about said plurality of layers by application of heat and pressure, said layers being suificiently plastic to accommodate relative movement between said conductor and said molded insulation.

3. An article of manufacture comprising an electrical conductor for carrying relatively high potentials and subjected to longitudinal expansion due to the heating effects of the current in said conductor, a plurality of layers of plastic rubber enveloping the lateral and end surfaces of said conductor, said rubber including a plasticizer for preventing the same from hardening and providing a strengthened skin surface for each of said layers, and a molded phenol con densation product enclosing the outermost one of said layers and having a relatively low coefficient of expansion, said plastic layers being resilient to provide for relative movement between said conductor and said molded condensation product.

4. The method of insulating an electrical conductor at relatively high potentials and subjected to expansion due to the heating effects of the current in said conductor, which comprises enclosing said conductor in a plastic envelope of adhering rubber, said rubber having a plasticizer incorporated therein for retaining it in a plastic condition and providing a strengthened outer skin surface, applying an insulating casing thereover comprising a molded phenol condensation product having a relatively low coefiicient of expansion as compared to said conductor, said intermediate rubber envelope accommodating relative movement between said conductor and said casing, and ageing said insulating casing to remove shrinking stresses therefrom.

5. The method of insulating an electrical conductor carrying current at relatively high potentials and subjected to expansion due to the heating effects of said current which comprises applying a series of layers of a solution consisting of gum rubber and a plasticizer about the conductor, applying an outer enclosing coating of a moulded phenol condensation product about said series of layers, and proportioning the relative thicknesses of the insulating coatings to produce a maximum voltage gradient for each insulating coating which will be below a value at which corona forms when the conductor is subjected to a definite operating voltage.

6. A transfer stud comprising a solid metallic conductor, a buffer comprising a plurality of substantially homogeneous layers of unvulcanized gum rubber disposed over said conductor, a body of insulation comprising a phenol condensation product molded around the conductor and buffer, said buffer being yielding to provide for relative movement between said conductor and said body of insulation, the outer surface of said rubber being toughened to prevent relative heat and pressure to produce a substantially rigid 15 body over said rubber.

JOSEPH C. RAH. 

